The skin is a vital organ composed of several layers (dermis, proliferative layers and stratum corneum) covering the entire surface of the body and essentially providing a barrier function with respect to the external environment. This barrier function relies especially on the quality of the epidermis which, in particular, depends on the stratum corneum's condition and on the balance between the proliferation and differentiation of the epidermal keratinocytes. The quality and appropriate functioning of the skin are closely related to the water content in the different layers of the epidermis. Thus, in normal epidermis, the proliferative layers contain about 70% water, whereas the stratum corneum contains only 10 to 15%.
Hydration of the stratum corneum is the result of three factors: the supply of water from the dermis, water loss to the outside environment, and the ability of the stratum corneum to bind water molecules.
The regulation of water distribution is carried out by hormones (aldosterone, sex hormones), pH or osmotic changes. Cell membranes are by nature hydrophobic and are therefore not highly permeable to water, although water channels exist, which are sorts of pores that facilitate the passage of water and certain solutes.
Aquaporins are a class of transmembrane proteins transporting water and small molecules in solution, such as glycerol and urea, which facilitate water transport in the epithelia and endothelia.
The key function of aquaporins in the epithelia involved in the transport of water or solutes, such as in the kidney, has been extensively studied (Deen et al., 1994). Similarly, aquaporins were rapidly identified in the saliva- or tear-producing exocrine glands. However, the discovery of type 3 aquaporin, or AQP3, in human skin, specifically in the plasma membrane of keratinocytes of the proliferative layers of the epidermis (SOUGRAT R. et al., J. Invest. Dermatol., 2002), highlighted the importance of a regulated flow of water into the skin. AQP3s can transport water and glycerol, with the latter playing an important role both in the formation of the surface hydrolipid film and in the preservation of the stratum corneum's flexibility and sensory qualities.
The importance of AQP3s has been demonstrated in mice. Indeed, the inactivation of the F gene causes multiple skin deficiencies, such as low moisture level, ineffective barrier function, increased tissue regeneration time and reduced elasticity (T. Ma et al., J. Mol. Biol., 2002). It has since been discovered that aquaporin 3 is expressed in dermal fibroblasts where it is involved in the migration of these cells during wound regeneration (Cao C. et al., Biochem J., 2006). On the other hand, aquaporins take part in the barrier function by positively regulating the establishment of cellular bonds and communications of the tight-junction type (Kawedia J. et al., PNAS 104(9), 2007).
AQP3 hydration and content in keratinocytes are closely linked. Thus, the increase in skin AQP3 improves hydration of the epidermis (M. Dumas, J. Drugs Dermatol., June, 2007).
Cutaneous water loss may have several origins, namely hereditary, acquired or related to the environment. In a very dry environment, water loss through evaporation from the stratum corneum is significant and may exceed the rate of replacement through transcellular diffusion.
During skin aging, the skin becomes dry. Thus, in elderly patients, in particular in those aged 50 and above, xerosis or dry mucous membranes associated with lower sebum secretion, hormonal changes or slowdowns of the water flow through the epidermis are often seen to occur. The skin is then subject to itching and tautness, two characteristic symptoms of dry skin. Examples of acquired conditions resulting in dry skin include photo-chemotherapy-induced xerosis and eczema. Examples of acquired diseases causing dry mouth, or xerostomia, include Sjogren's syndrome or neck radiation therapy. Lastly, examples of conditions involving mucosal dryness include vaginal or ocular dryness.
A first alternative treatment for dry skin is to administer topical products intended to restore the skin barrier, such as wetting agents capable of binding water, examples of which include urea and lactic acid entering in the composition of NMF (Natural Moisturizer Factor; proteolytic derivatives of filaggrin), film-forming agents for retaining water, or agents capable of rebuilding the skin barrier (squalene, ceramides, fatty acids,). However, these products have a superficial action which does not correct the biological defects of skin subjected to chronic dehydration.
In this context, as a result of their special properties, aquaporins are potential biological targets for improving skin hydration and reducing signs of skin dryness. Thus, patent FR 2 801 504 describes the increase of AQP3 in the skin by using an extract of Ajuga turkestanica plants and has resulted in improved skin hydration. A pomegranate extract has also been reported as an oral or topical active agent to stimulate aquaporin activity and regulate water and glycerol transport in tissues (FR 2 874 502). A total extract of carob pulp, at the same time containing proteins, carbohydrates and polyphenols and its use to moisturize the skin, has been described in French Patent 2 905 857, without the mechanism of action of such an extract being suggested.
On the other hand, a carob bean protein extract obtained through hydrolysis and intended for the production of a fertilizer easily assimilated by plants has already been disclosed (J. Parrado et al., Bioresource Technology 99, 2008). The disclosed extract is rich in low molecular weight peptides, but also sugars and phytohormones. However, phytohormones are polyphenolic compounds that modulate plant growth, but are considered as endocrine disruptors in humans because they are often likely to affect functions such as growth, development, behavior or production. The presence of such compounds is not desirable in an extract intended for a cosmetic or pharmaceutical application.
The present invention relates to a carob bean peptide extract containing peptides having a molecular weight of less than 5 kDa and having a very low polyphenol content (less than 1%) and a sugar content of less than 15%. This chemical composition provides the extract with a high degree of safety and results in a more targeted molecular activity. The inventors have thus found that the extract of the invention promotes the expression of aquaporins, in particular aquaporin 3.